The sign of work done by a force is important to understant. State carefully if the following quantities are positive or negative. (a) Work done by a man in lifting a bucket out of a well by means of a rope tied to the bucket. (b) Work done by the gravitational force in the above case. (c ) Work done by friction on a body sliding down an inclined plane. (d) Work done by an applied froce on a body moving on a rough horizontal plane with uniform velocity. (e ) Work done by the resistive force of air on a vibrating pendulum in bringing it to rest.

Explanation

(a) Positive , because bucket moves in the direction of applied force .
Negative , because bucket moves in the direction opposite to gravitational force .
(c ) Negative , because friction always opposes the relative motion , and the friction direction of displacement is opposite to the frictional force .
Positive , as the body moves in the direction of applied force .
(e ) Negative , because resistive force is opposite to the direction of motion of the bob .